This invention relates to a coated paper which is suitable as a recording sheet for use in an ink jet recording process. In particular, this invention relates to such a recording sheet which exhibits excellent curl and imaging behavior.
Ink jet printing systems generally are of two types, continuous stream and drop-on-demand. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components in an ink-filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high-quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is a thermal ink jet, or bubble jet, and produces high-velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again.
Ink jet printers of the continuous stream type employ printheads having one or more orifices or nozzles from which continuous streams of ink droplets are emitted and directed toward a recording medium. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. Printing information is transferred to the droplets of each stream by electrodes that charge the passing droplets, which permits each droplet to be individually charged so that it may be positioned at a distinct location on the recording medium or sent to the gutter for recirculation. As the droplets proceed in flight from the charging electrodes toward the recording medium, they are passed through an electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium. The continuous stream ink jet process is described, for example, in U.S. Pat. No. 4,255,754, U.S. Pat. No. 4,698,123, and U.S. Pat. No. 4,751,517.
Papers coated with materials compatible with ink jet inks are known. For example, U.S. Pat. No. 4,478,910 discloses an ink jet recording paper comprising a base sheet with a coating layer comprising a water-soluble polymeric binder and fine silica particles. The polymeric binder may include polyvinyl alcohol or its derivatives, water soluble cellulose derivatives, water soluble polymeric substances such as polyvinyl pyrrolidone, or the like.
U.S. Pat. No. 4,758,461 discloses a recording paper suitable for ink-jet comprising a fibrous substrate paper on the surface of which a silicon containing type pigment and a fibrous material of the substrate paper are present in a mixed state. The paper can also contain an aqueous binder such as one or a mixture of two or more water-soluble or water-dispersed polymers such as polyvinyl alcohol, starch, oxidized starch, cationized starch, casein, carboxymethyl cellulose, gelatin, hydroxyethyl cellulose, SBR latex, MBR latex, vinyl acetate emulsion, and the like.
U.S. Pat. No. 4,780,356 discloses a recording sheet suitable for ink jet printing comprising a sheet of paper and porous particles on the paper surface. The particles can be coated on a paper surface by means of a binder such as polyvinyl alcohol.
U.S. Pat. No. 4,474,847 discloses a coated base paper for use in an ink jet recording process wherein the coating comprises a pigment and/or filler of non-flake structure and a binding agent dried on the paper. The pigment content is at least about 90 percent by weight of the dried coating, and the binding agent is predominantly hydrophilic.
U.S. Pat. No. 4,686,118 discloses a recording medium having an ink receiving layer provided on a substrate. The ink receiving layer comprises at least a mixture of a polymer capable of forming intermolecular hydrogen bonds and a polymer incapable of forming intermolecular hydrogen bonds. The substrate can be opaque, e.g., paper, wood, metal plate, or transparent, e.g., polyester film.
U.S. Pat. No. 4,554,181 discloses an ink jet recording sheet having a recording surface which includes a combination of a water soluble polyvalent metal salt and a cationic polymer, said polymer having cationic groups which are available in the recording surface for insolubilizing an anionic dye.
U.S. Pat. No. 4,617,239 discloses a method of coating paper to improve its surface strength and printability by applying to the paper a silicon-containing modified polyvinyl alcohol agent or its saponification product. The coating agent forms a film on the surface of the paper which minimizes the penetration of the coating into the paper and improves the surface strength and printability of the paper. The coating agent may be incorporated with other coating compounds, including synthetic resin emulsions such as styrene-butadiene latex, polyacrylate ester emulsion, polyvinyl acetate emulsion, vinyl acetate-acrylate ester copolymer emulsion, and vinyl acetate-ethylene copolymer emulsion. Further, the coating agent may be incorporated with pigments such as clay, calcium carbonate, titanium dioxide, satin white, zinc oxide, silica, aluminum oxide, and cadmium sulfide.
U.S. Pat. No. 4,481,244 relates to a recording sheet which comprises a substrate and a coating layer formed thereon of a coating material containing a polymer having both hydrophilic segments and hydrophobic segments.
One of the important physical properties a paper recording medium should possess is the ability to remain flat over a wide range of humidities. Unacceptable curling of a recording medium over time can occur, particularly when the humidity in the surrounding environment changes. The root cause of curl in recording papers for ink jet processes is the differential in hydroexpansivity between the coating and the paper. The paper and coating absorb different quantities of water at a given humidity, thereby causing swelling or shrinking differentially, resulting in unacceptable curl with changes in humidity.
There are many well-documented methods employed to yield moisture sensitive materials having good curl characteristics. It is important, however, that any method used must not affect imaging performance. The physical properties, e.g., sheet stiffness and functional characteristics of the sheet, must not be compromised by the actions taken to yield a product with good curl characteristics.
A commonly used technique is the addition of a second coating to the backside of the recording sheet. This coating is applied to counteract any shrinkage or swelling resulting from the functional top coating. The back coating may generally include the same coating as the top coat, e.g., polyvinyl alcohol, full or partially hydrolyzed, various starches or carboxymethylcellulose. The addition of a second coating to balance the differential hydroexpansivity, however, adds cost and may not reduce the curl to the desired extent in cases where precise amounts of the two coatings are needed.
Applying an aqueous impermeable coating to both sides renders the base paper dimensionally stable, but curling due to shrinking and swelling of the functional coating will still occur. Moreover, the addition of this layer may greatly reduce the receptive ability for the aqueous inks.
Partial chemical cross-linking such as in water miscible urea formaldehyde systems can be used to improve curling characteristics. This mechanism would bind the water receptive polymer with the cross-linked network, yielding it unreactive. This technique would not remove the reactivity of the base paper, however, and has long cure times.
Ink jet recording papers therefore still have problems today in avoiding unacceptable curl over a wide range of humidities. The search continues for a recording paper useful in ink jet recording processes which exhibits excellent curl behavior over a wide range of humidities, but which also is economical and capable of excellent imaging.
An object of the present invention, therefore, is to provide a coated ink jet recording paper which avoids the problems of curl, particularly upon a change of relative humidity.
Another object of the present invention is to provide a paper recording medium for ink jet recording processes which has excellent imaging properties and exhibits excellent properties with respect to dimensional stability, specifically curl behavior.
Still another object of the present invention is to provide a recording paper useful in ink jet recording processes which is economically and easily manufactured while providing the desired imaging performance and curl behavior.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.